Characteristics which have been demanded for an aqueous ink for an aqueous ink jet printer in recent years include good fastness properties such as water resistance and light resistance of print images, no disadvantages such as an irregular flow of an aqueous ink irrespective of the kind of print medium and a larger spread than an aqueous ink droplet adhered (hereinafter referred to as “a blur”), and high print image density-color reproduction irrespective of the kind of print medium.
Of such purposes, for securing the fastness of print images, it has been studied that pigments excellent in fastness are utilized in place of dyes. Different from the dyes, the pigments have no solubility in water, so that it is necessary to disperse the pigments in water in a finely dispersed state. However, it is very difficult to stably keep this dispersed state. Accordingly, techniques for stably dispersing the pigments in water have been variously proposed. There has been proposed a method using a dispersible surfactant (for example, see patent document 1) or a dispersible polymer having a hydrophobic moiety and a hydrophilic moiety (for example, see patent document 2), as a means therefor. Further, as a method for coating a surface of a colorant with a polymer, there has been proposed a method using microcapsules encapsulating a dye aqueous ink as an aqueous ink for an aqueous ink jet printer (for example, see patent document 3), a method using microencapsulated coloring matter in which coloring matter dissolved or dispersed in a solvent insoluble in water is emulsified in water with a dispersible surfactant (for example, see patent document 4), a method using microcapsules in a recording solution, the microcapsules containing an inclusion in which a subliming disperse dye is dissolved or dispersed in at least one of water, a water-soluble solvent and a polyester (for example, patent document 5), an ink composition comprising colored emulsion-polymerized particles and an aqueous material (for example, patent document 6) or a method according to a phase reversal emulsification reaction or an acid precipitation method (for example, patent document 7).
Further, for realizing print images good in color reproduction, there has been proposed a method of performing image formation by combining a yellow aqueous ink, a magenta aqueous ink, a cyan aqueous ink and a black aqueous ink, in which specific pigments are each dispersed in a water-soluble resin (for example, see patent documents 8 and 9), or a method of performing image formation by combining a yellow aqueous ink, a magenta aqueous ink, a cyan aqueous ink, a black aqueous ink, a green aqueous ink and a red aqueous ink, in which pigments are each dispersed in a water-soluble resin (for example, see patent document 10).
However, the conventional dispersions are unstable, and the presence of the substance having a hydrophobic moiety and a hydrophilic moiety such as a surfactant or a glycol ether results in the easy occurrence of adsorption-desorption, which causes the problem of inferior storage stability of the aqueous ink. In order to reduce blurs to paper, an ordinary aqueous ink necessitates the substance having a hydrophobic moiety and a hydrophilic moiety such as a surfactant or a glycol ether. An aqueous ink not using such a substance becomes insufficient in permeability to paper. In order to perform uniform printing, the kind of paper is limited, and there has been the problem that the deterioration of print images becomes liable to occur.
Further, when an additive as used in the invention (an acetylene glycol, an acetylene alcohol, a silicone surfactant, di(tri)ethylene glycol monobutyl ether, (di)propylene glycol monobutyl ether or a 1,2-alkylene glycol, or a mixture thereof) is used in the conventional dispersion, long-term storage stability is not obtained, and the re-solubility of the aqueous ink is inferior. Accordingly, the aqueous ink has the problem that it is dried, resulting in easy clogging at a nozzle of an aqueous ink jet head or a pen point of a writing material.
Further, when the pigment is dispersed by such a dispersing agent, the residual of the dispersing agent remains in an aqueous ink system, and the dispersing agent does not sufficiently contribute to dispersion to move from the pigment, which caused the problem of increased viscosity. When the viscosity is increased, the added amount of the colorant such as the pigment is limited. Accordingly, sufficient image quality is not obtained, particularly in plain paper.
Then, in order to solve the above-mentioned problems, specifically in order to provide an aqueous ink containing a dispersion which makes it possible to prepare an aqueous ink realizing reduced blurs and high color development on plain paper and having a fixability in addition to sufficient color development on exclusive paper, and which makes it possible to prepare an aqueous ink further excellent in ejection stability in aqueous ink jet recording, the present inventors have disclosed an aqueous ink containing a dispersion in which a colorant is encapsulated with a polymer having aromatic rings in an amount of 20 to 70% by weight to make it dispersible in water (for example, see patent document 11).
Patent Document 1: JP-A-1-301760
Patent Document 2: JP-B-5-64724
Patent Document 3: JP-A-62-95366
Patent Document 4: JP-A-1-170672
Patent Document 5: JP-A-5-39447
Patent Document 6: JP-A-6-313141
Patent Document 7: JP-A-10-140065
Patent Document 8: JP-A-5-155006
Patent Document 9: JP-A-10-52925
Patent Document 10: JP-A-2001-354886
Patent Document 11: Pamphlet of WO03/033602
However, one described in the above-mentioned patent document 11 has not been satisfactory in fixability and ejection stability.